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Abstract:

An exhaust system for a vehicular lean burn internal combustion engine
that emits oxides of nitrogen (NOx) and particulate matter (PM) is
disclosed. The system comprises a NOx reduction catalyst for reducing NOx
in the presence of a reductant, means for introducing reductant into a
flowing exhaust gas and a source of hydrocarbon reductant, a nitrogenous
reductant, and/or hydrogen, a filter for removing PM from exhaust gas
flowing in the exhaust system and a low pressure exhaust gas
recirculation (EGR) circuit for connecting the exhaust system downstream
of the filter to an air intake of the engine. The EGR circuit comprises a
IMOx adsorber catalyst (NAC) comprising a NO adsorbent.

Claims:

1. An exhaust system for a vehicular lean burn internal combustion engine
that emits oxides of nitrogen (NOx) and particulate matter (PM), the
system comprising a NOx reduction catalyst for reducing NOx in
the presence of a reductant, means for introducing reductant into a
flowing exhaust gas and a source of reductant selected from the group
consisting of a hydrocarbon reductant, a nitrogenous reductant, hydrogen
and mixtures of any two or more thereof, a filter for removing PM from
exhaust gas flowing in the exhaust system and a low pressure exhaust gas
recirculation (EGR) circuit for connecting the exhaust system downstream
of the filter to an air intake of the engine, wherein the EGR circuit
comprises a NOx adsorber catalyst (NAC) comprising a nitric oxide
(NO) adsorbent.

2. An exhaust system according to claim 1, wherein the NO adsorbent
consists of palladium and a cerium oxide or a mixed oxide or composite
oxide containing cerium and at least one other transition metal.

3. An exhaust system according to claim 1, wherein the NO adsorbent
includes palladium dispersed on ceria or a mixed oxide or composite oxide
containing cerium and at least one other transition metal.

4. An exhaust system according to claim 2, wherein the at least one other
transition metal in the mixed oxide or composite oxide is zirconium.

5. An exhaust system according to claim 1, wherein the NO adsorbent has a
palladium loading ranging from 0.1 to 200 g/ft.sup.-3.

6. An exhaust system according to claim 1, wherein the NO adsorbent is
combined with a thermally regenerable NOx adsorbent for net
adsorbing NOx at above 200.degree. C. and net desorbing NOx at
above 250.degree. C., which thermally regenerable NOx absorbent
comprising platinum and a metal oxide.

8. An exhaust system according to claim 6, wherein the NO adsorbent is
present in an underlayer and the thermally regenerable NOx absorbent
is present in a layer overlying the underlayer.

9. An exhaust system according to claim 1, wherein the NOx reduction
catalyst is located downstream of the filter.

10. An exhaust system according to claim 9, wherein the means for
introducing reductant into a flowing exhaust gas is located between the
filter and the NOx reduction catalyst.

11. An exhaust system according to claim 1, wherein the NOx
reduction catalyst is located on the filter.

12. An exhaust system according to claim 11, wherein the means for
introducing reductant into a flowing exhaust gas is located upstream of
the filter.

13. An exhaust system according to claim 1, comprising an oxidation
catalyst for oxidising NO to nitrogen dioxide located upstream of the
filter and/or the NOx reduction catalyst.

14. An exhaust system according to claim 1, wherein the NOx
reduction catalyst selectively catalyzes the reduction of NOx using
a nitrogenous reductant.

15. An exhaust system according to claim 1, wherein the nitrogenous
reductant is ammonia, hydrazine or an ammonia precursor selected from the
group consisting of urea ((NH2)2CO), ammonium carbonate,
ammonium carbamate, ammonium hydrogen carbonate and ammonium formate.

16. An exhaust system according to claim 1, comprising means for
controlling the introduction of reductant into the exhaust gas in order
to reduce NOx therein.

17. An exhaust system according to claim 16, wherein the control means
comprises an electronic control unit.

18. An exhaust system according to claim 16, wherein the control means
comprises a NOx sensor located downstream of the NO reduction
catalyst.

19. A lean-burn internal combustion engine comprising an exhaust system
according to claim 1.

20. A compression ignition engine according to claim 19.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to an exhaust system for a vehicular
lean burn internal combustion engine that emits oxides of nitrogen and
particulate matter (PM).

BACKGROUND OF THE INVENTION

[0002] Exhaust gas recirculation (EGR) is a method of reducing emissions
of oxides of nitrogen (NOx) from an engine by returning a portion of
an engine's exhaust gas the engine combustion chambers via the air
intake. EGR works by lowering the oxygen concentration in the combustion
chamber, thereby decreasing the peak temperature of the fuel combustion
flame as well as through heat absorption. EGR is not a new technology--it
has been used since the mid-1970s in gasoline fueled passenger car
engines. Following the gasoline application, EGR was also introduced to
diesel passenger cars and--from the early 2000s--to heavy-duty diesel
engines.

[0003] Generally, there are two exhaust system arrangements comprising
EGR: (i) high pressure loop EGR, in which the exhaust gas is recirculated
from upstream of a turbocharger to ensure that exhaust gas will flow from
the former to the latter; and (ii) low pressure loop EGR (also called
long loop EGR), where exhaust gas is often recirculated from downstream
of a particulate filter, allowing all the exhaust gas to be utilised in
the turbo. Exhaust gas pressure downstream of the filter is generally
lower than at the intake manifold, allowing exhaust gas to flow from the
former to the latter location.

[0004] In use, particularly during cold start in a vehicle configured to
meet the MVEG-A drive cycle, an EGR valve is set to recirculate
approximately 50% of the exhaust gas to the engine. Exhaust gas emitted
from the engine during EGR has a lower oxygen content but a no higher
NOx content than exhaust gas recirculated from the exhaust system to
the engine.

[0005] WO 2008/047170 discloses a method of reducing oxides of nitrogen
(NOx) present in a lean gas stream comprising nitric oxide (NO), the
method comprising the steps of: (i) net adsorbing NO per se from the lean
gas stream in an adsorbent comprising palladium and a cerium oxide at
below 200° C.; (ii) thermally net desorbing NO from the NO
adsorbent in a lean gas stream at 200° C. and above; and (iii)
catalytically reducing NOx on a catalyst other than the NO adsorbent
with a reductant selected from the group consisting of a hydrocarbon
reductant, a nitrogenous reductant, hydrogen and a mixture of any two or
more thereof. There is also disclosed a system for reducing NOx in a
lean gas stream comprising NO, which system comprising an adsorbent for
adsorbing NO per se from the lean gas stream at below 200° C.,
means for contacting the NO adsorbent with a lean gas stream at
200° C. and above thereby to desorb NO from the NO adsorbent and
means for reducing NO desorbed from the NO adsorbent comprising a NO
reduction catalyst and a source of reductant selected from the group
consisting of a hydrocarbon reductant, a nitrogenous reductant, hydrogen
and mixtures of any two or more thereof, wherein the NO adsorbent
comprises palladium and a cerium oxide.

[0006] We now propose an exhaust system arrangement that can improve
NOx conversion over a legislative drive cycle (such as the European
MVEG-A drive cycle) and in real world conditions which can lower the
NOx emissions from vehicular lean burn internal combustion engines
relative to current commercial exhaust system arrangements.

SUMMARY OF THE INVENTION

[0007] The invention includes an exhaust system for a vehicular lean burn
internal combustion engine that emits oxides of nitrogen (NOx) and
particulate matter (PM), and a lean-burn internal combustion engine
containing the exhaust system. The system comprises a NO reduction
catalyst and a source of reductant selected from the group consisting of
a hydrocarbon reductant, a nitrogenous reductant, hydrogen and mixtures
of any two or more thereof, a filter for removing PM from exhaust gas
flowing in the exhaust system and a low pressure exhaust gas
recirculation (EGR) circuit for connecting the exhaust system downstream
of the filter to an air intake of the engine. The EGR circuit comprises a
NO adsorber catalyst (NAC) comprising a NO adsorbent. An advantage of
this arrangement is that NO reduction catalysts often are inactive for
NOx reduction at temperatures below about 200° C.
Additionally, where the NOx reduction catalyst reduces NOx
using a urea reductant, it may not be possible to inject urea until the
exhaust gas is sufficiently warm enough to decompose urea into CO2
and ammonia, otherwise it risks clogging the downstream NOx
reduction catalyst and/or the urea injector with solid deposits of urea.
By adsorbing NO in an exhaust gas recirculation circuit at temperatures
lower than a desired light-off temperature of the NOx reduction
catalyst before thermally i.e. passively, releasing stored NO back into
exhaust gas when the NOx reduction catalyst is active for NOx
reduction, the system as a whole emits less NOx, increasing design
options for the skilled person to meet a relevant emission standard.

BRIEF DESCRIPTION OF THE DRAWING

[0008] FIG. 1 is a schematic flow diagram of one embodiment of the
invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The invention is an exhaust system that comprises a NO reduction
catalyst and a source of reductant selected from the group consisting of
a hydrocarbon reductant, a nitrogenous reductant, hydrogen and mixtures
of any two or more thereof, a filter for removing PM from exhaust gas
flowing in the exhaust system and a low pressure exhaust gas
recirculation (EGR) circuit for connecting the exhaust system downstream
of the filter to an air intake of the engine. The EGR circuit comprises a
NOx adsorber catalyst (NAC) comprising a NO adsorbent.

[0010] The filter is preferably a wall-flow filter.

[0011] In one embodiment, the NO adsorbent consists of palladium and a
cerium oxide or a mixed oxide or composite oxide containing cerium and at
least one other transition metal, whereas in another embodiment, the NO
adsorbent includes palladium dispersed on ceria or a mixed oxide or
composite oxide containing cerium and at least one other transition
metal. In a particular embodiment, the at least one other transition
metal in the mixed oxide or composite oxide is zirconium.

[0012] Suitable palladium loadings in the NO adsorbent can be from 0.1 to
200 g/ft-3 (0.0035 to 7.1 g/L). In one embodiment, the palladium
loading on the cerium oxide or the composite oxide containing cerium is
from 0.1 to 200 g/ft-3 (0.0035 to 7.1 g/L), such as from 0.5 to 150
g/ft-3 (0.018 to 5.3 g/L) or 1 to 120 g/ft-3 (0.035 to 4.2
g/L).

[0013] "Composite oxide" as defined herein means a largely amorphous oxide
material comprising oxides of at least two elements which are not true
mixed oxides consisting of the at least two elements.

[0014] Materials comprising palladium and a cerium oxide can be obtained
by known methods including, but not limited to, impregnation, the
incipient wetness technique and co-precipitation. Co-precipitated
materials can be obtained according to the methods disclosed in EP
0602865.

[0015] In another embodiment that can be used, though not exclusively, in
treating NOx emitted from lean-burn internal combustion engines,
particularly vehicular engines, the NO adsorbent is combined with a
thermally regenerable NOx adsorbent for net adsorbing NOx at
above about 200° C., and thermally net desorbing NOx at above
250° C. (i.e. net adsorbing from above about 200° C. up to
about 250° C.), which thermally regenerable NOx absorbent
comprising platinum and a metal oxide. In embodiments, the thermally
regenerable NOx absorbent can comprise platinum dispersed on alumina
and/or zirconia. An advantage of this embodiment is that it enables net
NOx desorption to be delayed to higher temperatures (temperatures
above about 250° C., such as 255° C., 260° C.,
265° C. or 270° C.), at which a relevant NO reduction
catalyst (e.g. a selective catalytic reduction catalyst using nitrogenous
reductant) is more active (i.e. above light off temperature) for NO
reduction, than at temperatures immediately above 200° C., or
which enables ammonia precursors such as urea--which hydrolyzes or
pyrolyzes more readily at higher temperatures--to be used.

[0016] As used herein the terms "absorb" and "adsorb" and any derivatives
thereof have been used interchangeably, and the specification should be
interpreted accordingly.

[0017] The NO adsorbent and the thermally regenerable NOx absorbent
can be disposed in any suitable arrangement that provides this benefit.
For example, in one embodiment a flow-through substrate monolith is
coated with a zone at an inlet end of the substrate monolith with the NO
adsorbent and at an outlet end of the substrate monolith with the
thermally regenerable NOx absorbent. Alternatively, the NO adsorbent
is present in an underlayer on a substrate monolith and the thermally
regenerable NOx absorbent is present in a layer overlying the
underlayer.

[0018] Suitable NOx reduction catalysts known in the art include lean
NOx catalysts (also known as hydrocarbon-SCR catalysts), which can
use hydrocarbon and/or hydrogen as reductant, or a NOx trap
comprising an alkaline earth metal or alkali metal NOx adsorber
component and a NO oxidation catalyst component, suitably comprising
platinum, and optionally a NOx reduction catalyst component, such as
rhodium. In a preferred embodiment, NOx reduction catalyst is a
selective catalytic reduction (SCR) catalyst and the reductant is a
nitrogenous reductant.

[0019] The filter and NOx reduction catalyst can be arranged in any
suitable configuration. In one embodiment, the NOx reduction
catalyst is located downstream of the filter. In this embodiment the
means for introducing reductant into a flowing exhaust gas is suitably
located between the filter and the NOx reduction catalyst, but may
also be located upstream of the NOx reduction catalyst if
arrangements are made to avoid combustion of the reductant on the filter.

[0020] In another embodiment, the NOx reduction catalyst is located
on the filter. Where the filter is a wall-flow filter, the NOx
reduction catalyst can be formulated as a washcoat that permeates the
walls of the filter. This can be done, for example, by milling the
catalyst to an average particle size of ≦5 μm. In this
embodiment the means for introducing reductant into a flowing exhaust gas
is suitably located upstream of the filter.

[0021] In a preferred embodiment, an oxidation catalyst for oxidising NO
to nitrogen dioxide located upstream of the filter and/or the NOx
reduction catalyst.

[0022] Preferably, the NOx reduction catalyst selectively catalyzes
the reduction of NOx using a nitrogenous reductant. Suitable
selective catalytic reduction catalysts include transition metal promoted
molecular sieves such as aluminosilicate zeolites and
silicoaluminophosphates. Suitable transition metal promoters include Cr,
Ce, Mn, Fe, Co, Ni and Cu and mixtures of any two or more thereof.
Preferred molecular sieve catalysts include CuCHA, such as Cu-SAPO-34,
Cu-SSZ-13, and Fe-Beta zeolite, where either the Fe is present in the
framework of the molecular sieve structure and/or otherwise associated
e.g. ion-exchanged with the framework structure. Fe--WOx-ZrO2 can be
used as a active non-molecular sieve SCR catalyst.

[0023] The nitrogenous reductant for use in the system can be ammonia per
se, hydrazine or an ammonia precursor selected from the group consisting
of urea ((NH2)2CO), ammonium carbonate, ammonium carbamate,
ammonium hydrogen carbonate and ammonium formate.

[0024] The reductant for use in the system or method according to the
invention is a suitable hydrocarbon reductant, nitrogenous reductant or
hydrogen. In arrangements employing hydrocarbon reductant in the
preferred use of the system according to the invention, the hydrocarbon
reductant can be a hydrocarbon fuel that powers the engine. Where the
reductant is a hydrocarbon fuel, it may be desirable to crack the fuel to
form shorter chain hydrocarbons in order to promote more efficient
NOx reduction. In this regard, Pd/CeO2 is a particularly
efficient catalyst for cracking hydrocarbon fuel.

[0025] Nitrogenous reductants can include ammonia per se, hydrazine or an
ammonia precursor selected from the group consisting of urea
((NH2)2CO), ammonium carbonate, ammonium carbamate, ammonium
hydrogen carbonate and ammonium formate. Hydrogen can be generated in
situ for example by contacting a hydrocarbon fuel with a suitable
reformer catalyst or, where the gas comprises carbon dioxide and water,
by contacting the gas stream with a suitable water-gas shift catalyst.

[0026] The reductant is added to the flowing exhaust gas by any suitable
means for introducing the reductant into the exhaust gas. Suitable means
include an injector, sprayer, or feeder, and is preferably an injector.
Such means are well known in the art.

[0027] The system may comprise means for controlling the introduction of
reductant into the exhaust gas in order to reduce NOx therein. In
one embodiment, the control means comprises an electronic control unit,
optionally an engine control unit. Furthermore, the control means may
comprise a NOx sensor located downstream of the NO reduction
catalyst.

[0028] According to a further aspect, the invention provides a lean-burn
internal combustion engine comprising an exhaust system according to the
invention. The lean-burn internal combustion engine can be a lean-burn
gasoline or a diesel engine, but the engine may also run on alternative
fuels such as liquid petroleum gas, natural gas or comprise bio-fuels or
gas-to-liquid products. In a particular embodiment, the lean-burn
internal combustion engine is a compression ignition engine powered e.g.
by diesel fuel.

[0029] In order that the invention may be more fully understood, the
following Examples are provided by way of illustration only and with
reference to the accompanying drawing. cl EXAMPLE

[0030] FIG. 1 is a schematic diagram of a vehicular lean-burn internal
combustion engine comprising an exhaust system according to a first
embodiment of the invention featuring a thermally regenerable NO
adsorbent disposed in an exhaust gas recirculation circuit.

[0031] Referring to FIG. 1, there is shown a diesel engine 12 comprising
an exhaust system 10 according to the present invention. The exhaust
system comprises an exhaust line 14 wherein aftertreatment components are
disposed in series. The NO oxidation catalyst 16 comprises a ceramic
flow-through substrate monolith coated with a NO oxidation catalyst
composition comprising a platinum rich combination of platinum and
palladium supported on an alumina-based high surface area support
material.

[0032] A ceramic wall-flow filter 20 comprising a washcoat of Cu-SSZ-13
selective catalytic reduction catalyst is disposed downstream of NO
oxidation catalyst 16. An ammonia oxidation clean-up or slip catalyst 36
is coated on a downstream end of the SCR catalyst monolith substrate.
Alternatively, the ammonia slip catalyst can be coated on a separate
substrate located downstream of the SCR catalyst (not shown). Means
(injector 22) is provided for introducing reductant fluid (urea 26) from
reservoir 24 into exhaust gas carried in the exhaust line 14. Injector 22
is controlled using valve 28, which valve is in turn controlled by
electronic control unit 30 (valve control represented by dotted line).
Electronic control unit 30 receives closed loop feedback control input
from a NOx sensor 32 located downstream of the SCR catalyst.

[0033] Low pressure exhaust gas recirculation circuit 17 comprises an
exhaust gas recirculation valve 18 also controlled by the electronic
control unit 30. Disposed within the exhaust gas recirculation circuit,
NO adsorbent 19 comprises a ceramic flow-through substrate monolith
coated with a NO adsorbent composition comprising palladium supported on
ceria that net adsorbs NO from lean exhaust gas at up to about
200° C. and net desorbs NO in lean exhaust gas at temperatures
above about 200° C.

[0034] In use, the palladium supported on ceria NO adsorbent 19 net
adsorbs NO from exhaust gas flowing in exhaust gas in the exhaust gas
recirculation circuit at temperatures of up to about 200° C., for
example following cold start in the MVEG-A European drive cycle or during
driving conditions that produce cooler exhaust gas, e.g. extended periods
of idling in traffic. As the exhaust gas temperatures in the exhaust gas
recirculation system rise, NO is thermally (i.e. passively) desorbed and
passes to the engine intake and, following emission from the engine, a
proportion of NOx derived from desorbed NO is reduced on the SCR
catalyst 20 in the presence of ammonia derived from urea injected via
injector 22. The ammonia slip catalyst 36 oxidises NH3 that would
otherwise be exhausted to atmosphere.